1. Field of the Invention
The present invention generally relates to heat exchangers for use in regulating the temperature of a patient's blood during surgery. More particularly, the invention concerns micro-conduit with enhanced wetting characteristics for use in the heat exchanger portion of a blood oxygenator.
2. Description of the Related Art
"Heart-lung" machines are known in the medical field. One component of these machines is a blood oxygenator. Blood oxygenators arc typically disposable and serve to oxygenate a patient's blood during medical procedures such as heart surgery. Most blood oxygenators employ a gas transfer membrane, which comprises thousands of tiny hollow fibers having microscopic pores. Blood flows around the outside surfaces of these fibers while a controlled oxygen-rich gas mixture flows through the fibers. Also, due to the relatively high concentration of carbon dioxide in the blood arriving from the patient, carbon dioxide in the blood diffuses through the fibers' microscopic pores and into the gas mixture. Due to the relatively low concentration of oxygen in the blood arriving, from the patient, oxygen from the gas mixture diffuses into the blood through the fibers' microscopic pores.
Most blood oxygenators also employ a heat exchanger to precisely regulate the temperature of a patient's blood. The heat exchanger usually includes one or more conduits housed in a vessel. The patient's blood is continuously pumped through the conduits, while a heat exchange medium such as water flows through the vessel around the conduits, or vice versa. The heat exchange medium is either heated or cooled to maintain the patient's blood at a desired temperature.
One example of a commercially successful blood oxygenator is sold under the designation MAXIMA.RTM. by Medtronic Corp. In the MAXIMA blood oxygenator, the heat exchange medium (water) blood flows inside relatively large diameter metal tubes while blood flows on the outside of the tubes within the vessel. The TERUMO brand oxygenator uses a different configuration, where blood flows inside relatively large, diameter metal tubes. In the BARD WILLIAM HARVEY HF-5700 blood oxygenators the blood flows outside plastic tubes that contain a flow of temperature-regulated water.
Known heat exchangers, such as those described above, have benefited the purposes of doctors and patients alike in many different applications. However, in their quest to continually update these products, design engineers are always seeking improvements. In this regard, some applications may benefit from heat exchangers with improved heat exchange characteristics. One sure way to increase the heat exchange rate is to increase the area of contact between the blood and heat exchange medium. Enlarging the heat exchanger, however, is undesirable because of the greater bulk, the increased weight, and the enlarged blood volume required to fill the vessel.
A different way to improve the heat exchange efficiency is to increase the number of heat exchanger tubes while decreasing their size. Furthermore, by running, blood inside these smaller tubes, the blood is more thoroughly and evenly exposed to the heat exchange medium. This approach can be problematic because priming small diameter tubes can be difficult or impossible. When "priming" the heat exchanger tubes, typically an aqueous solution is pumped through the tubes to displace the ambient air. Then, blood is pumped into the tubes following the aqueous solution. Priming small plastic tubes is problematic because the tubes tend to act as capillary tubes, which are associated with certain undesirable properties. In particular, the use of a large number of tubes has the effect of reducing the pressure drop across the tubes. Therefore, if a few tubes become plugged with bubbles of air or water, the aqueous priming solution easily finds a path through other unplugged tubes. Consequently, the trapped bubbles reluctantly remain in the plugged tubes. There can be dangerous consequences if an air bubble somehow releases and subsequently passes into the patient's bloodstream. Therefore, lack of reliable priming can be a significant problem in heat exchangers with a large number of small diameter heat exchanger tubes.
Another problem with the above approach is finding an appropriate material for manufacturing the heat exchanger tubes. Many materials are not suitable for this application due to the obviously low tolerance to contamination and toxicity. Although metals have been successfully used in past blood heat exchangers, metals present a number of difficulties. First, since tubes of small diameter must be manufactured more precisely, they are more expensive than larger tubes. Furthermore, this expense is compounded due to the increased number of heat exchanger tubes required in this design.
Unlike metals, plastics are typically inexpensive. However, many desirable plastics have relatively low critical surface tension, and thus do not "wet" easily with water or other aqueous priming solutions. This aggravates the priming problems resulting from the capillary effect in small diameter tubes, as discussed above. Furthermore, plastics have poor heat transfer characteristics and therefore their use necessitates an even greater surface area to efficiently achieve the desired heat exchange.